I'm trying to type wedding invitation envelopes which are 5x7 and I don't know how to create the format. In the pages area, the available templates are all standard envelope sizes. How do I create the 5x7 format template

I presume you are using the Pages application, is that correct?

So start off by launching Pages and choose one of the regular envelope styles. Traditional works well for me.

Next go to the File menu and select Page setup. From the menu that says Paper Size select Manage Custom Sizes. In the resulting dialog, there should be a button on the left labeled "+". Click it. In the Paper Size area, select Width 5 in and Height 7 in. Name the size (where it says "untitled" something descriptive like "5 x 7 envelope". Click the OK button. The Paper Size menu here should now say the description you gave it. Click OK again. Your work area should now be a 5 x 7 envelope.

Best of luck.

PS If you are using Word or some other program, the process is probably the same or very similar.

In addition, if you go to File > Save as Template, you won't need to do the resizing every time.